Compared with other existing batteries, lithium ion batteries and other lithium secondary batteries are small and lightweight, and are endowed with a high energy density and an excellent power density. As a result, in recent years, such batteries have been advantageously used as portable power sources for personal computers and handheld electronic devices, and as power sources for driving vehicles.
One implementation of this battery is the sealed lithium secondary battery. This battery is typically fabricated by introducing an electrode assembly made up of a positive electrode and a negative electrode, together with an electrolyte (typically an electrolyte solution), into a battery case, then attaching a lid to the case and closing (sealing) the case in a very low humidity environment (referred to below as a “dry environment”). The electrodes (the positive electrode and the negative electrode) are each formed of an electrode mixture layer (positive electrode mixture layer and negative electrode mixture layer) composed primarily of an active material which can reversibly intercalate and deintercalate the charge carriers (lithium ions) on the corresponding positive or negative current collector.
However, in a sealed lithium secondary battery, the trace amount of moisture present in these battery components and a portion of the electrolyte ingredients (nonaqueous solvent, supporting salt, etc.) are decomposed at the electrode surfaces during initial charging, resulting in the generation of gases. The generation of such gases causes the internal pressure of the battery to rise. Related art that addresses this problem is described in Patent Literature 1 to Patent Literature 3. For example, Patent Literature 1 discloses art in which the battery is initially charged before the battery case is sealed, the gases generated with charging are discharged outside of the battery case, and then the case is sealed, Patent Literature 2 discloses a method which includes the steps of first pre-sealing the battery case in a dry environment, then initially charging the battery in open air, and subsequently discharging the generated gases and carrying out a main sealing operation in, once again, a dry environment.